extra print from a class function exported to python using pybind11

Question

Here is the example.cpp code :

#include <pybind11/pybind11.h>
namespace py = pybind11;


int add(int i, int j) {
    return i + j;
}

class mycl{
    public:
    mycl(){};
    mycl(int a){
        age = a;
    };
    void call(){
    printf("I'm here!\n");
    }
    int ask_age(){
        printf("my age is %d\n", age);
    }
    private:
    int age;
};

class mycl myclv;
class mycl *pmycl = &myclv;

PYBIND11_MODULE(example, m) {
    m.doc() = "pybind11 example plugin"; // optional module docstring

    using namespace pybind11::literals;
    m.def("add", &add, //"A function which adds two numbers",
         "i"_a, "j"_a);
         //py::arg("i"), py::arg("j"));
    py::object omycl = py::cast("Pointer to mycl");
    m.attr("ptr") = omycl;
    py::class_<mycl>(m, "mycl")
    .def(py::init<int>())
    .def("AskAge",&mycl::ask_age);
}

I compiled this as below (I have pybind11 module here) :

c++ -std=c++11 example.cpp -fPIC -shared `python3 -m pybind11 --include` -o example.so

Here is the python3 execution result :

ckim@chan-ubuntu:~/PYBIND11/pybind11$ python3
Python 3.5.2 (default, Jul 17 2020, 14:04:10) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import example
>>> a = example.mycl(53)
>>> a.AskAge()
my age is 53
13
>>> b = example.mycl(54)
>>> b.AskAge()
my age is 54
13
>>> 

I don't know where the 13 is coming from. What is the problem?

Answer 1

C++ printf returns the total number of characters printed (see here). You bind AskAge() to ask_age which returns an int. Since printf also returns an int, I guess that is whats causing the interprenter to print 13, since the sentence my age is 54 is 13 characters long (with the \n at the end).

If you would run this program not from the interprenter, it would not return 13.